Impedance measuring bridge utilizing delta bridge networks for balancing out spurious effects



Sept. 12, 1967 JULIE 3,341,773

IMPEDANCE MEASURING BRIDGE UTILIZING DELTA' BRIDGE NETWORKS FORBALANCING OUT SPURIOUS EFFECTS Original Filed Sept. 8, 1964 2Sheets-Sheet l INVENTOR WJM' Sept. 12, 1967 JULIE 3,341,773

' IMPEDANCE MEASURING BRIDGE UTILIZING DELTA BRIDGE NETWORKS FORBALANCING OUT SPURIOUS EFFECTS Original Filed Sept. 8, 1964 2Sheets-Sheet 2 I 24 Z5 Z6 V/ ..i i J E 5; Zu 35 1 q i I o g 62- I I 35a55 I i E a;

I I l l V; L.. J

29 3/ 60 7 V4 '4', A I

2 a; f Z5 lNVENTOR BY%WJM ATTORNEY United States Patent 3,341,773IMPEDANCE MEASURING BRIDGE UTILIZING DELTA BRIDGE NETWORKS FOR BALANC-ING OUT SPURIOUS EFFECTS Loebe Julie, New York, N.Y., assignor to JulieResearch Laboratories, Inc., New York, N.Y., a corporation of New YorkContinuation of application Ser. No. 394,932, Sept. 8, 1964. Thisapplication June 22, 1966, Ser. No. 559,694 4 Claims. (Cl. 324-57) Thisapplication is a continuation of application Serial No. 394,932, filedSept. 8, 1964, now abandoned.

The instant invention relates to improvements in a direct reading ratioset and, in particular, a network comprising a mul-ti-arm measuringbridge having a three terminal voltage divider as one arm of the bridge.

As known in the art, a direct reading ratio set is made up of acombination of fixed and adjustable resistors arranged in the form of aWheatstone measuring bridge. Two arms of the bridge are selected to formthe ratio set to provide extremely accurate ratio arms for comparisonmeasurements and substitution tests of resistors. In the conventionalratio set, a first arm is usually selected to be a fixed value of 100ohms; the second arm is a variable resistance providing discrete stepsfrom about 99.445 ohms to 100.555 ohms.

In one method of using such bridge, an auxiliary resistor of the samenominal value as the resistors under comparison serves as the thirdbridge arm. The resistors under test are interchangeably connected intothe fourth arm wherein the bridge is balanced separately each time thefourth arm resistor is changed. The bridge is balanced by varying theratio of the variable arm with respect to the fixed arm. The differencein the ratio for two balances gives the percentage difference betweenthe two fourth arm resistors under test. Thus, if the difference in theratios for the two balances is one step on the 0.001 dial of the movablearm, one resistor is 0.001% different than the other. For example, if1,000-ohm resistors are compared, the difference of 0.001% of 1,000 ohmsis 0.01 ohm.

In another method of prior art bridge operation, the standard is placedin the third arm and the unknown of nominal and similar value to thestandard is placed in the fourth arm. A balance is obtained and then thepositions of the standard and unknown are interchanged,

whereby a second balance is obtained. The two balances are averaged todetermine the percentage variation of the unknown with respect to thestandard.

When the unknown and standard resistors under comparison arecharacterized by low values of resistance, for example, values less than100 ohms, and where extremely high precision comparison results aredesired, contact,

lead wire and bridge loading effects present formidableproblems whichmake high precision comparison measurements in the order of a few partsper million diflic-ult, if not impossible, to obtain. The four terminalconnection is often used to minimize the spurious effects of lead andcontact resistances. However, when employing four terminal yokes in theconventional ratio set circuits, a variety of numerous balances arerequired. As a simple example, first balances using current terminalsare made, then balances using voltage terminals are made and then eachof the voltage terminals at the standard interconnections are made. Thenumber of balances required for a complete comparison measurement inaccordance with the prior art systems becomes unduly involved,particularly, where a large number of resistors are required forcomparison measurements.

Additional problems arise when measurements of resistors withinaccuracies of a few parts per million (p.p.m.)

are attempted. In the first place, the state of the art bridges do notprovide such accuracies with sufficient reliability and consistency. Forexample, even after a state of the art bridge is suitably calibrated, a1 milli-ohm variation in contact resistance, as one is interchanging theresistors under measurement, will introduce an error of 10 parts permillion with respect to the measurement of -ohm resistors, and it isrecognized that variations in contact resistances of 1 milli-ohm are notuncommon.

My co-pending application Serial No. 195,680, filed May 18, 1962, nowPatent No. 3,179,880, issued Apr. 20, 1965, and entitled ImpedanceMeasuring Apparatus illustrates and claims a three-terminal voltagedivider in a closed loop bridge for providing high accuracy precisionresistor measurements Within the absolute value of l or 2 parts permillion.

It is the principal object of the instant invention to extend theapplication of the invention set forth in the aforesaid application as adirect reading ratio set.

It is a further object of the invention to provide a closed bridge loopdirect reading ratio set employing a three terminal voltage divider ascontemplated in the aforesaid application as one arm of the bridge andincorporating composite lead, contact and load compensation yokes at twosensitive junctions of the bridge so as to balance out all spuriouseffects that might arise during bridge balance measurements, whereinresistance comparisons and measurements in the order of 1 part permillion in absolute values are achieved.

It is a further object of the invention to provide a direct readingratio set which is automatically self-calibrating with respect to itsfixed resistor arm and the variable arm comprising the three terminalvoltage divider while the sensitive low impedance circuit of the bridgeis completely isolated therefrom during such calibration to therebyincrease the precision and accuracy of the bridge calibration.

It is a further object of the invention to provide a direct readingratio set as described hereinbefore capable of automatic andself-compensation against lead, contact and other yoke effects at thesensitive junctions of the bridge, wherein variations of any of suchspurious eliects during bridge use are automatically divided intosuitable bridge arm ratios as the bridge is actually being balanced, soas to eliminate any errors that otherwise would be introduced by suchspurious effects.

It is a further object of the invention to provide a bridge measuringsystem as contemplated herein for providing a direct 1:1 range of ratiomeasurements and by mere switching operation an equally accurate 10:1range of ratio measurements.

It is a further object of the invention to provide a direct readingratio set for accurate high precision ratio measurements, wherein thereadings of bridge balance will indicate at least to 1 part per millionthe actual difference of the resistor under test with respect to astandard.

It is a further object of the invention to provide a direct readingratio set, wherein the low resistance sensitive portion of the circuitis isolated against physical change and variations during bridgeoperation, and such sensitive portion of the system remains physicallyconstant during bridge operation so as to minimize contact, lead andload resistance effects.

Further objects and advantages will become apparent from the followingdescription of the invention taken in conjunction with the figures, inwhich:

FIG. 1 illustrates schematically a four terminal connection between tworesistors at a junction of a closed bridge loop;

FIG. 2 illustrates schematically the equivalent electrical circuit ofFIG. 1;

FIG. 3 is a schematic of a Wheatstone bridge circuit employing fourterminal connections at the junctions thereof;

FIG. 4 is an expanded Y formation of the delta formation illustrated inFIG. 2;

FIGS. 5 and 6 illustrate expanded Y formations at the four junctions ofthe bridge depicted in FIG. 3; and

FIG. 7 is an electrical schematic of a direct reading ratio set inaccordance with the invention.

Four terminal lead connections are employed when dealing with highprecision and extreme accuracy measurement and calibration circuitrywith respect to resistors characterized by low values of resistance.Four terminal lead connections serve to minimize the error effects ofsuprious lead wire and terminal contact resistances. My co-pendingapplication Ser. No. 562,045, filed June 6, 1966 and entitled AccurateImpedance Measuring Apparatus depicts a four terminal connection asapplied to resistors in a bridge. Reference may be made to saidapplication for further details concerning such connections.

The instant invention contemplates the use of a closed loop directreading ratio bridge adapted for the precision measurement andcalibration of low value resistors. The four terminal junctionconnection forms a yoke. FIG. 1 illustrates such connection at thejunction of two resistors a and c of a bridge. Reference numbers 11 and12 depict the conductive terminal ends of the resistors. Reference 13depicts the connection to the common junction with the bridge cross-armconnected to resistors a and c. Line 70 depicts one lead connection ofthe four-terminal yoke between said two resistors. Lines 71 and 72depict the respective second connections from each resistor a, c to thecommon junction 13 of one of the bridge cross-arms. The equivalentschematic circuit of FIG. 1 is depicted in FIG. 2. The end of resistor ais at point 11. The end of resistor c is at point 12. The commonconnection or tap to the cross-arm of the bridge is depicted at 13. Theresistances 70', 71' and 72 depict the respective equivalent resistancescorresponding to the actual contact and lead wire resistances generatedby the yoke terminal connection at the junction of resistors a, c. Asseen from FIG. 2, the yoke formed at the junction of resistors a and cis a delta formation. Merely for illustrative purposes, FIG. 3 depictsschematically a closed-looped bridge having four terminal yokeconnections at its junctions. The bridge arms are depicted as a, b, cand d.

When balancing the bridge in accordance with the principles of theinvention, a composite balance is achieved. That is to say, the arms ofthe bridge are balanced to provide a proper ratio in order to null anindicator meter M and, at the same time, contact and lead resistors suchas 70, 71 and 72 are also balanced in the same ratio as the respectiveadjacent bridge arms. After the bridge is suitably balanced, a normallyclosed switch 14, shown schematically in FIG. 2 and in series with yokeresistance 70, may be opened without affecting the null of meter M orbridge balance. The purpose of switch 14 will become apparenthereinafter.

FIG. 2 depicts schematically the delta yoke connection at the junctionof resistors a, c. Assume hereinafter with respect to FIG. 2 that switch14 is closed. The delta of FIG. 2 can be converted into an equivalent Y,depicted in FIG. 4. Point 11 is the connection with the end of resistora. The equivalent Y resistance i is a series continuation of theresistance of resistor a. Point 12 is the connection to the end ofresistor c. The equivalent Y resistance j is a series continuation withresistor c. The equivalent Y resistance 73 is in series with point 13and becomes a continuation of or part of the cross-arm of the bridge.Using four yokes at the junctions of the four bridge arm resistors a, b,c, d and converting each delta yoke formed at such terminal junctionsinto resistive Y formations, one achieves the equivalent circuit of FIG.5.

For the moment, it will be understood that each of the four bridge armsare of low resistance value to justify the four terminal yokeconnections. By reason of the Y conversion, each bridge arm includesrespective yoke resist ances i, j, k, l, m, etc., in series therewith,which yoke resistances are the equivalent of the contact and lead wireresistances of the yoke connections. For example, resistor a has contactand lead Wire resistances i and l in series therewith, as shown, to forma total bridge arm resistance of a. Resistor c has contact and lead wireresistances j and n in series and at the opposite ends thereof to form atotal bridge arm resistance of 0. Similarly, bridge arm b is made up ofresistor b in series with yoke resistances k and p; and bridge arm d ismade up of resistor d in series with yoke resistances 0 and m. Theforegoing equivalent bridge is depicted in FIG. 5

When all four main bridge arms of FIG. 5 are balanced, the ratio of theyoke resistances including all contact and lead resistances at anyjunction are in the same ratio to their respective sides; for example:

The foregoing ratio is explained by the fact that for balance, thevoltage potentials from junction 74 to junctions 11 and 12,respectively, are the same. Consequently, to maintain bridge balance,the current ratios through arm a and through arm 0 must be the same asthat if resistance i and j were zero. With the addition of resistances iand j in series with their respective sides, the ratio of i and j shouldbe the same as the ratio of the total resistance of the two arms if thesame current ratios are to be maintained to continue bridge balance. Inother words, the proper current ratios between the currents through aand c and, thus, bridge balance is maintained if Equation 1 holds true.

Equation 1 may be written as:

a w c H (2) where symbol H is a ratio parameter. Similarly, it can beshown that:

where symbols G, F and E are ratio parameters as defined in Equation 3.From Equations 2 and 3, the yoke resistances can be relabeled andexpressed as follows:

But, from FIG. 6:

and

d=d'(l-F-G) (7) Substituting Equation 7 into Equation 5 provides:

LFG)(L-EH d LEF)(L-GH provides:

at l

The error term in bridge balance from Equation 9 is defined as:

(H F) G E In accordance with the principles of the invention, the directreading ratio setting as contemplated herein is constructed to reducethe value of the error term (H-F) (G-E), whereby bridge error becomesnegligible.

FIG. 7 illustrates schematically, an embodiment of the invention. Onearm of the bridge, depicted as Zd, includes a three terminal voltagedivider 40 provided with a pair of input terminals 15, 16 and an outputterminal 59. Voltage divider 40 is characterized by a selected value offixed input impedance and a linear transfer ratio regulated by a sixdial setting. Divider 40 has a six digit transfer ratio dial 54 forregulating the divider transfer ratio. The theory and operation of abridge employing such voltage divider means as one arm of the loopbridge, is the subject matter of my co-pending application Ser. No.195,680, filed May 18, 1962, now Patent No. 3,179,- 880, issued Apr. 20,1965, and entitled Impedance Measuring Apparatus. Reference may be madeto said application for the basic theory and operation of the bridge ascontemplated in FIG. 7.

The bridge arm Zc in FIG. 7 serves as an arm of selected value of fixedresistance, whereas Zd comprises the variable ratio arm. When the testresistors Za and Zb are characterized by values of low resistance, eachwill have a four terminal yoke connection; two for current and two forvoltage. Symbols I and I depict the two current connections for resistorZa. Symbols V and V depict the two voltage connections for resistor Za.Similarly, resistor Zb has two current connections I and I and twovoltage terminals V and V The interconnection between the two testresistors Za, Zb is a lead and compensation yoke resistance circuit.Current connections I and I are connected through a normally closedswitch 17. The Voltage connections V and V are connected through aseries resistor yoke circuit comprising resistors 18, 19, and 21. Forthe illustrative example, the values of the foregoing series resistorsare: R =100 ohms; R is a 1 ohm potentiometer; R :90 ohms; and R 10 ohms.The purpose of these resistors and the illustrative values there of willbecome apparent in subsequent discussion.

The yoke formed between resistors Za and Zb is an expanded deltaformation of the Y connection between resistors a and b of FIGS. 5 and6. The ratio of the yoke resistances l and k to the values of therespective total bridge arm resistances of the bridge for bridgebalance,

. 6 is controlled by the ratio parameter E. Thus, as shown hereinbefore,at null, yoke resistances l and k will be divided between the two armsZ0 and Zb in the same ratio as the respective total values of such twoarms.

Resistor 19 includes a movable tap contact 22 which connects through acontact terminal 58b of switch 58 to a null detector 23 in one of thebridge cross-arms. The other side of detector 23 is connected to theoutput terminal 59 of voltage divider 40.

In accordance with the principles of the invention, resistor Za isconnected through a lead and contact resistance compensation yokearrangement to bridge arm Zc. This arrangement involves connectingcurrent terminal I through series resistors 24, 25 and 26 to a junction27 at the upper end of arm Zc. For the disclosed example, the values ofthe foregoing series resistors are as follows: R is a 1 ohmpotentiometer; R =10 ohms; and R =l00 ohms.

Potential terminal V of resistor Za is connected through a 1.1 ohmvariable load compensation resistor 28 and a normally closed seriesswitch 53 to junction 27. The delta yoke formed between resistor Za andarm Zc can be converted into the Y yoke as shown schematically in FIGS.5 and 6, which in said figures said yoke appears between resistors a andc. This yoke formation would divide the contact resistances and otherseries resistances comprising the total i and j by the ratio parameterH. Thus, at a null, the yoke resistances i and j are placed in theappropriate bridge arms to maintain the total bridge arm ratio a to c tomaintain bridge balance.

The invention also contemplates a second yoke between the Zb and Zdarms. The potential terminal V, of resistor Zb is connected through aseries circuit of a load compensation resistor 29 of 0.05 ohms and anormally closed switch 30 to a junction 31 at the lower end of arm Zd.Current terminal 1 of resistor Zb is connected through added lead andcontact compensation series resistors 32, 33 and 34 to junction 31. Theillustrative values of the aforesaid series resistors are: R is a 1 ohmpotentiometer; R =10 ohms; and R ohms. The total yoke formed betweenresistors Zb and Zd, shown in delta formation in FIG. 7, may beconverted to an equivalent Y as shown in FIGS. 5 and 6. This yoke isbetween resistors b and d of FIGS. 5 and 6, and the ratio of totalresistances p and 0 to the correlated bridge arms b and d will be inaccordance with the ratio parameter P.

A normally opened shorting switch 35 is placed in parallel across theseries connected bridge arms Z0 and Zd. Switch 35 is connected betweenjunctions 27 and 31.

From FIG. 6, ratio parameter G is formed by the yoke between bridge arms0' and d. The actual bridge circuit of FIG. 7 does not use a fourterminal yoke between arms Z0 and Zd. As illustrated hereinafter, theresistance values of Zc and Zd are established very accurately byself-calibration means incorporated in the bridge circuit. Furthermore,the resistance values of arms Z0 and Zd are each relatively large. Thesefactors eliminate the need of a four terminal connection between arms20, Zd. As a result, the connection between these two arms is aconventional two terminal connection, as depicted in FIG. 7, therebyreducing the ratio parameter G to a value of zero. When the parameter Gis equal to zero, the error term, Equation 10, becomes:

.ized by a 1:1 range and, in particular, keeping within the i parts permillion accuracies as contemplated herein, the values of arms Zc and Zdshould be exactly equal. In the illustrated embodiment, the value of Zis selected to be 100 ohms. The value of Zd is selected to be a variableresistance in the range of 100 ohms, and which can be set to equal Zcexactly. Voltage divider 40 serves as the variable arm for the ratioset. The input impedance of divider 40 is selected to be a fixed valueof 100,000 ohms. A precision and highly stable matching resistor 41 isselected to have a value equal to the input impedance of divider 40,hence a value of 100,000 ohms is connected in series with divider 40. Aresistor 42 is connected in parallel across the series combination ofdivider 40 and matching resistor 41. Resistor 42 is shunted by a fixedresistor 43 in series with a variable trimmer resistor 44. Theparameters for resistors 41, 42 and 43 are selected to provide a stableprecision resultant resistance of 100 ohms. Trimmer 44 provides anadjustment whereby the resultant value of Zd, terminal 15 to junction31, exactly equals Zc. Consequently, the Zd bridge arm, that is to say,from terminal 15 to junction 31 is essentially a combination of 200,000ohms in parallel with 100 ohms which is effectively 100 Ohms. By meansof such arrangement, a variation of the dial setting of the six dialvoltage divider 40 provides readings of values directly in parts permillion.

The fixed resistance arm 20 is made up of a precision and high stabilityresistor 39 having a selected value of 100 ohms in parallel with aprecision resistor 45 selected to have a value of 200,000 ohms. Thus,the combination making up Zc will match exactly the value of Zn. Anymismatch occurring betwen these two arms is eliminated by adjustment oftrimmer 44.

It will be noted that yoke resistances 25, 26 are provided withrespective shorting switches 25a, 26a. Similarly, resistances 33, 34 areprovided with respective shorting switches 33a, 34a. As seenhereinbefore, Zc and Zd have values in the order of 100 ohms each.Assume that Za of ohms is being compared to Zb of 10 ohms. To satisfythe bridge balance equations, in particular Equation 1, the yokeresistance ratio i/j and p/o for the foregoing measurement should bedivided by a ratio 1:10 at both junctions. This requires loading the twoyokes at the foregoing ratio and is accomplished by shorting outresistors 26, 34 while retaining resistors 25, 33. If one were tocompare Za, Zb having resistances in the order of 1 ohm each, the yokeresistances should be divided at a 1:100 ratio. This is provided byshorting out resistances 25, 33 and retaining resistances 26, 34 in thecircuit.

The direct reading ratio set of FIG. 7 is also capable of providing a10:1 range by means of a series of ganged switches 49, 50, 51 and 52, asdisclosed in FIG. 7. The conversion also requires the addition ofsuitable matching resistors in the circuit as follows. In the bridge armZc, a precision and highly stable resistor 46 having a selected value of1,000 ohms is connected across resistor 39 and a resistor 47 having aselected value of 1.1 megohms is connected across resistor 45. In thebridge arm Zd, a matching resistor 48 selected to have a value of 10,000ohms is connected across resistor 41. Switches 49 to 52 are providedwith correlated switch terminals 1' and r When switches 49 to 52 contactrespective terminals r the 1:1 range is obtained. Switch 52 is connectedto the dummy contact terminal r thereby maintaining resistor 20 in theseries circuit of the Za-Zb yoke, whereby the 90-ohm resistor 20 is inseries with the 10-ohrm resistor 21 to provide a resultant 100 ohms tomaintain a 1:1 ratio with the 100-ohm resistor 18. Potentiometer 19serves as a trimmer.

When the ganged switches 49 to 52 are moved to their respective terminalpositions, r the circuit now provides the 10:1 range. For this switchingposition resistor 20 is shorted out and resistors 39, 41 and 45 areinactive and resistors 46, 47 and 48 are in their respective circuits.However, a further adjustment is required to provide the 10:1 range. Forexample, for the 10:1 range, Zc is in the order of 1,000 ohms and Zd isin the order of ohms. Suppose the ratio set is operated to compare Za=l0ohms and Zb=1 ohm, this requires a yoke resistance division at the upperand lower junctions of 1:100. That is to say, i/ and 17/0 should bedivided into the ratios of 1:100. By shorting out resistors 25, 33 andretaining resistors 26, 34, the foregoing distribution of yokeresistances are obtained. If one is comparing Za=l00 ohms to Zb:10 ohms.resistors 26, 34 are shorted out and resistors 25, 33 are retained.

In order to achieve the high level of accuracy and precision ascontemplated herein, the direct reading ratio set is calibrated toprovide a precise ratio value for Z0 and Zd. For example, when thesystem is used for the 1:1 range, the value of Zc should be exactlyequal to the value of Zd and when operating in the 10:1 range, the ratioof these two bridge arms should be exactly equal to 10:1. One method forproviding such precision bridge arm calibration includes selecting twoextremely precision standard resistors at least substantially equal invalue and placing them interchangeably in the positions of Zn and Zb,wherein the bridge is repeatedly balanced until the bridge null meter 23indicates a constant reading. This technique, however, is not completelysatisfactory because the contact and lead resistance yokes would stillprovide slight inaccuracies whenever the two standard resistances areinterchanged between positions Zn and Zb.

FIG. 7 illustrates a technique for self-calibration incorporated intothe bridge. A pair of precision standard resistors 55 and 56, each10,000 ohms in value, are in series across junctions 27, 31. A leadconnection 57 from the mid-point between resistors 55, 56 connects toswitch contact terminal 5811. It will be understood when switch 58contacts its terminal 5812, the bridge circuit operates as a comparisonset for measuring a resistor Za against Zb. When switch 58 is moved toposition 58a, resistors 55, 56 serve as the calibrating bridge arms. Theother two arms are the actual bridge arms Z0 and Zd which are now beingcalibrated. Movement of switch 58 and 58a puts meter 23 in seriesbetween divider output 59 and the mid-point of resistors 55, 56. Thehigh resistance value of the 10K resistors 55, 56 eliminates the needfor using four terminal connections thereat. Accordingly, the entirecalibration circuit comprises standard two terminal connections. Dividerdial 54 is set either for l or 0 depending upon the polarity ofterminals 15, 16 in the circuit. Let us assume that divider terminal 15is the high side and divider terminal 16 the low side and the dividerinput is hooked into the circuit as shown in FIG. 7. Divider transferratio dial 54 is then set for l. The bridge is balanced by adjustingtrimmer 44. After null is indicated, resistors 55, 56 are interchangedand battery 38 is reversed. This is achieved by ganged switches 60, 61and 62, 63. Switches 60, 61 are at the opposite ends of battery 38.Switches 62, 63 are at the outer ends of resistors 55, 56. By reason ofoperation of these switches, resistor 55 is now at position 56 andresistor 56 is now at position 55. The negative terminal of battery 38is at the upper end and the positive terminal of battery 38 is at thelower end. The meter indicator is observed. Meter 23 will indicate thesame reading as the previous null if resistors 55, 56 are exactly equaland if Z0 is exactly equal to Zcl. On the other hand, if meter 23reading has changed from the first null position, the change ordeviation will be small. Trimmer 44 is adjusted to reduce the observeddeviation from the prior null reading to about one-half. Switches 6063are again actuated to interchange resistors 55, S6 and reverse battery38 to return same to the status shown in FIG. 7. The meter indication isagain observed to note if there is a deviation from the last adjustedreading. If a deviation still exists, trimmer 44 is again adjusted toreduce the deviation to about onehalf from the last adjusted reading.Once again, resistors 55, 56 are interchanged and battery 38 is reversedby switches 6063. This procedure is repeated until the meter readingremains constant for two successive readings. When such status isreached, Zc exactly equals Zd even if the absolute values of resistors55, 56 are slightly different.

The foregoing calibration of Zc and Zd was carried out withoutdisturbing in any way the four terminal yoke connections of resistors Zaand Zb, As a matter of fact, the actual resistors to be compared may bein their respective positions of Za and Zb while calibrating Z and Zd.Consequently, it is seen that one major advantage of the foregoingcalibrating circuit is that the low value resistors Za and Zb and theyokes thereof are essentially isolated from and remain constant duringcalibration of Z0 and Zd.

It is the purpose of the system to compare a high precision resistorwith a precision standard of known value, where Za or Zb may be theunknown and the other the known standard. If these resistors are notalready in the circuit while Zc and Zd are being calibrated, they arenow inserted therein. The bridge is now balanced to determine the valueof the unknown with respect to the standard.

The foregoing described calibration satisfies the values of Zc and Zdfor a 1:1 range operation. Calibration of the Z0 and Zd arms for the :1range is automatic for the following reason. The precision and highstability resistor constituting the l00-ohm Zc resistor 39 is actuallymade up of a series-parallel combination of ten precision and highlystable l00-ohm resistors. The same ten resistors are used to constitutethe precision and high stability 1,000- ohm Zc resistor 46 when switch50 moves to its position r This arrangement automatically provides anaccurate ratio of Zc=l0Zd for the 10:1 range after the arms arecalibrated for Zc=Zd for the 1:1 range. The technique of employingseries-parallel combination of precision and high stability resistors tomake up both the 100-ohm resistor 39 and the 1,000-ohm resistor 46 isknown in the art.

After calibrating Zc to equal Zd, switch 58 is returned to contact 58b.The set position of trimmer 44is not disturbed during the subsequentbridge balancing and measuring operation. The resistors Za and Zb are intheir bridge positions. The ratio divider dial 54 is set to provide atransfer ratio of 1.0.

The first bridge balance step involves balancing the bridge for null byadjusting dial 54 until meter 23 indicates a null. The null setting fordial 54 is not disturbed for the remaining sequence of four steps.

The second bridge balance step involves balancing the voltage ratio yokebetween arms Za and Zb. Switch 17 is temporarily opened. If a perfectbridge balance exists at this yoke, then the yoke impedances at thisjunction are properly divided in the same ratio as the respective bridgearms to which the yoke is connected, where-by opening of switch 17 willnot cause a bridge unbalance. On the other hand, if meter 23 indicatesan unbalance upon opening switch 17, potentiometer arm 22 is regulatedto restore bridge balance. Switch 17 is then returned to closed status;arm 22 remains at its last set position, and the third bridge balancestep is made.

The third step involves temporarily closing switch 35 to short out armsZc and Zd. A normally opened switch 35a is also temporarily closed toinsert meter M in the bridge loop described hereinafter. Thistemporarily establishes a bridge loop made up'of Za and Zb (theresistors under comparison) and the upper and lower yoke impedances,depicted by elements j and 0 in FIG. 5. If a perfect balance exists, itmeans that H :F, otherwise, a small meter deviation from balance will beobserved, whereby potentiometer 28 is adjusted and set to return thebridge to balance as indicated by meter 23, and then switch 35 is openedfor the next step. When the bridge is balanced for the four main arms:

a'/b'=c'/d' (12) If the bridge is also balanced when switch 35 isshorted,

But, the last equation holds if H =F, because by combintween Zb and Zd.Switch 30 is temporarily opened. If

meter 23 indicates an unbalance by reason of opening switch 30, balanceis restored by adjusting potentiometer,

arm 37. Potentiometer 37 remains at the set position and switch 30 isreturned to closed status.

The fifth balancing step is substantially similar to the fourth, exceptit involves balancing the upper yoke between arms Za and Z0. This ischecked out by opening switch 53 and balancing the bridge by regulatingpotentiometer arm 36, after which switch 53 is closed.

It will be understood that the fourth and fifth steps are essentiallybalancing the respective upper and lower yokes to provide the properyoke impedance ratios, as noted with respect to the discussionconcerning FIGS. 2 and 4. For balance at these junctions, the potentialsfrom junction 74 to junctions 11 and 12 are equal, assuming the nullmeter M is bridging arms a and c as shown in FIG. 3. Hence, the dropfrom junction 13 to junctions 11 and 12 are equal at bridge balance inaccordance with the ratio formula of Equation 1. Consequently, atbalance, opening and closing of switch 14 (FIG. 2) will not disturb thenull indication. In the circuit of FIG. 7, if the upper and lower yokesare in proper ratios, opening and closing switches 30 and 53 will notdisturb bridge balance. 7

After going through steps 1 to 5, it is preferable to repeat the bridgebalance sequence at least once more. For example, the last setting ofdial 54 is adjusted should meter 23 indicate a null deviation to returnthe bridge to balance. Then, steps 2 through 5 are repeated as outlinedhereinbefore.

The foregoing sequence of steps is one of a variety of balancingsequence steps that may be made. An example of another sequence forbalancing the bridge of FIG. 7-consists of steps 1 and 2 as describedhereinbefore, then followed by steps 4 and 5, and then concluded by step3,. which consists of closing shorting switch 35 and adjusting forbridge balance, if required, by regulating potentiometer 28.

The final reading of dial 54 indicates the ratio between Zr: and Zb. Forexample, if Za (the precision standard) is exactly 10 ohms and Zb is9.99998 ohms, the dial will read 999998. If Za and Zb were reversed, thearms Zc and Zd would have to be reversed since divider 40 does notprovide negative readings. When negative readings are indicated, arms Zcand Zd are interchanged by ganged switches 64, 65. Note, these switchesare at the outer ends of the high impedance elements of arms Z0 and Zd.When these switches are operated, resistor 45 or 47, as the case may be,is flipped into arm Zd across the ohms combination including resistor42; and the series combination of divider 40 and resistor 41 or resistor48, as the case may be, is flipped into arm Zc across resistor 39 or 46,as the case may be, so as to provide positive readings of the divider tobalance the bridge. The other alternative would be to interchange armsZa and Zb, but then the bridge has to be rebalanced.

The foregoing circuit may be used to provide a direct reading of theabsolute deviation of the resistor under test, for example, Zb from theprecision standardZa. In the above example the deviation was .00002 ohm.This merely requires hooking divider 40 into its circuit in reversedpolarity. For the above example, it was assumed that terminal 15 was thehigh potential side of divider 40 and terminal 16 was the low potentialside. Note, that junction 27 is the high potential side by reason of theillustrated battery polarity. On the other hand, if the high side ofdivider 40 is at terminal 16 and its low side is at terminal 15, anddivider dial 54 is set at 000000 for bridge calibration of Zc=Zd and forthe start of the first step of main bridge balance, the final reading ofdivider dial 54 will read directly the difference between Zb and Za,i.e. 0.00002. Such use of divider 40 is accurate for situations where aprecision resistor, such as Zb, is being calibrated against a knownprecision resistor and the deviation will be less than 1,000 p.p.m.,i.e. 0.001 ohm. Accordingly, we see a further advantage of the foregoinginvention, wherein the three terminal divider may be used to readdirectly the exact difference of the unknown with respect to theprecision standard.

The direct reading ratio set of FIG. 7 incorporates upper and loweryokes between ZaZc, and ZbZd. These yokes introduce two correctivecircuits into the system to eliminate bridge unbalance inaccuracies. Theindividual elements of the yoke networks are automatically divided up inproper ratios while the bridge is being balanced as arm Za is beingcompared to arm Zb to eliminate contact, lead and load resistanceinaccuracies. In the last analysis, these individual yoke elements formcomposite networks, whereby the bridge error term of the balanceEquation 11 reduces to zero. Furthermore, should variations in lead,contact or load effects arise during the actual bridge balancingoperation at any of the sensitive junctions, these yokes continuouslyapportion such spurious effects in proper ratios with respect tocorrelated bridge arms while bridge arms are being balanced to sustainbridge operation free of error, whereby reliable bridge measurements inthe order of 1 p.p.m. are maintained. It will also be noted, thatresistor Za, Zb and the other sensitive low resistance elements of thebridge remain physically constant during bridge calibration andbalancing operation. This is a further advantage of the system becauseit avoids bridge balance errors due to lead, contact and load resistancevariations, which occur when such sensitive portions of the bridge arephysically changed.

It is intended that all matter contained in the above description orshown in the accompanying drawings shall be interpreted as illustrativeand not in a limiting sense.

What is claimed is:

1. The method of measuring an impedance in a bridge comprising:

four bridge arms, a pair of bridge cross arms, power source means andnull indicator means in respective bridge cross arms, terminal endsconnected to the ends of said bridge arms, means including first, secondand third impedance networks in delta array for providing respectivebridge loop junctions, said first and second networks having respectiveequivalent Y impedances j and and junctions for interconnectingcorrelated pairs of bridge arms and for connecting a correlated crossarm, adjacent terminal ends of first and second of said bridge armsbeing interconnected by said first delta network, adjacent terminal endsof third and fourth of said bridge arms being interconnected by saidsecond delta network, said first and second delta networks also beingconnected to a first of said cross arms,

adjacent terminal ends of said first and third bridge arms beinginterconnected by said third delta network, one of said first and thirdbridge arms being an impedance under measurement,

the impedances of said second and fourth bridge arms being c'j and d0respectively, the said second and fourth arms and the said Y impedancesof their connected delta networks having tthe ratio parameters meansconnecting adjacent terminal ends of said second and fourth bridge arms,

a voltage divider providing a substantially constant impedance andhaving a pair of input terminals connected across one of said second orfourth bridge arms, said voltage divider also having a variable transferratio and an output terminal,

means for connecting the other bridge cross arm between said voltagedivider output and said third delta network,

switching means for applying temporarily a short circuit across saidsecond and fourth bridge arms, and

variable impedance means in at least one portion of said first andsecond delta networks for adjusting the balance of said temporary bridgeso that the ratio parameters (HF)=0;

including the steps of:

(1) first balancing the bridge by varying the transfer ratio of saidvoltage divider until said null indicator means indicates a null, and

(2) temporarily closing said switching means and adjusting the saidvariable impedance means of at least one of said first and second deltanetworks until a null is obtained and H =F.

2. The method of claim 1 in which the said third delta network alsocomprises adjustable impedance means and switching means, the step ofthe opening and closing of said second mentioned switching means and theadjusting of its adjustable impedance means until the opening andclosing of its switching means retains the bridge balance as shown by anull on the null indicator means.

3. The method of claim 1 in which the said first and second deltanetworks each has adjustable impedance means and switching means in eachof the delta networks, the steps of the opening and closing of theswitching means of the first delta network and the adjusting of itsadjustable impedance means until the opening and closing of itsswitching means retains the bridge balance as shown by a null on thenull indicator means, and

the opening and closing of the switching means of the second deltanetwork and the adjusting of its adjustable impedance means until theopening and closing of its switching means retains the bridge balance asshown by a null on the null indicator means.

4. The method of claim 1 in which the bridge also includes a pair ofseries connected resistances each having an outer terminal end and eachhaving impedance values very much greater than the impedance values ofany one of the four bridge arms, means for connecting said outerterminal ends of said series connected resistances to respectiveterminal ends of said second and fourth bridge arms for forming acalibrating bridge, and means for balancing said calibrating bridge forestablishing equal impedance values for said third and fourth bridgearms.

References Cited UNITED STATES PATENTS 1,665,397 4/1928 Wunsch 324622,589,758 3/1952 Wojciechowski 32457 2,954,929 10/1960 Dennis 323-- XR3,179,880 4/1965 Julie 32457 3,249,866 5/1966 Barr et al. 32462 RUDOLPHV. ROLINEC, Primary Examiner.

WALTER L. CARLSON, Examiner.

E. E. KUBASIEWICZ, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,341,773 September 12, 1967 Loebe Julie It is hereby certified thaterror appears in the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below.

Column 3, lines 6, 8, 64, 66,68, 70 and 74, column 4, line 3, column 5,line 72, column 6, lines 23 and 43, and column 11, lines 50 and 66, for"Y", each occurrence, read wye column 3, line 17, for "suprious" readspurious column 4, line 29, for "resistance" read resistances samecolumn 4, lines 72 to 74, and column 5, lines 1, l2, l4 and 23, for thelower-case letter "1", in italics, each occurrence,

read figure 1 column 5, lines 4, 5, 6 and 7, for

"L" read l column 6, line 64, for "-E-(H-FJ/D" read E (H-F) column 7,line 61, for "r read r Signed and sealed this 5th day of November 1968.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. EDWARD J. BRENNER Attesting Officer Commissionerof Patents

1. THE METHOD OF MEASURING AN IMPEDANCE IN A BRIDGE COMPRISING: FOURBRIDGE ARMS, A PAIR OF BRIDGE CROSS ARMS, POWER SOURCE MEANS AND NULLINDICATOR MEANS IN RESPECTIVE BRIDGE CROSS ARMS, TERMINAL ENDS CONNECTEDTO THE ENDS OF SAID BRIDGE ARMS, MEANS INCLUDING FIRST, SECOND AND THIRDIMPEDANCE NETWORKS IN DELTA ARRAY FOR PROVIDING RESPECTIVE BRIDGE LOOPJUNCTIONS, SAID FIRST AND SECOND NETWORKS HAVING RESPECTIVE EQUIVALENT YIMPEDANCES J AND O AND JUNCTIONS FOR INTERCONNECTING CORRELATED PAIRS OFBRIDGE ARMS AND FOR CONNECTING A CORRELATED CROSS ARM, ADJACENT TERMINALENDS OF FIRST AND SECOND OF SAID BRIDGE ARMS BEING INTERCONNECTED BYSAID FIRST DELTA NETWORK, ADJACENT TERMINAL ENDS OF THIRD AND FOURTH OFSAID BRIDGE ARMS BEING INTERCONNECTED BY SAID SECOND DELTA NETWORK, SAIDFIRST AND SECOND DELTA NETWORKS ALSO BEING CONNECTED TO A FIRST OF SAIDCROSS ARMS, ADJACENT TERMINAL ENDS OF SAID FIRST AND THIRD BRIDGE ARMSBEING INTERCONNECTED BY SAID THIRD DELTA NETWORK, ONE OF SAID FIRST ANDTHIRD BRIDGE ARMS BEING AN IMPEDANCE UNDER MEASUREMENT, THE IMPEDANCESOF SAID SECOND AND FOURTH BRIDGE ARMS BEING C''-4 AND D''-ORESPECTIVELY, THE SAID SECOND AND FOURTH ARMS AND THE SAID Y IMPEDANCESOF THEIR CONNECTED DELTA NETWORKS HAVING THE RATIO PARAMETERS